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Fundamentals of Cutaneous Photobiology and Photoimmunology at a Glance
  • When radiation enters the skin, it is scattered or absorbed. Only light absorbed by molecules (chromophores) in the skin can cause a photobiologic response.
  • Electromagnetic radiation can be conceptualized either as a wave or as packets of energy called photons.
  • An action spectrum indicates which wavelengths produce a photobiologic response most effectively and is plotted as the reciprocal of the minimum effective fluence versus wavelength.
  • The most erythemogenic wavelengths present in sunlight are in the ultraviolet B (UVB) range. Ultraviolet A (UVA) is roughly 1000-fold less effective than UVB.
  • Prostaglandins and nitric oxide appear to be the major mediators for UVB erythema.
  • When certain drugs and dyes absorb UV/visible light, inflammation ensues. This is called photosensitization.
  • Photosensitivity responses are usually mediated by reactive oxygen species.
  • Blocked by sunscreen use, cutaneous vitamin D production is mediated by wavelengths from 295–300 nm. Optimal vitamin D blood levels are essential for good bone health and increasingly associated with a myriad of other potential health benefits.
  • Ultraviolet radiation is immunosuppressive. Local and systemic immunosuppressions are recognized.
  • Pyrimidine dimers, reactive oxygen species and urocanic acid initiate UV-induced immunosuppression.
  • Interleukin-10 (IL-10), tumor necrosis factor (TNF)-α, platelet-activating factor, platelet-activating factor-like lipids and other molecules mediate photoimmunosuppression.
  • The major cellular players in UV immunosuppression are Langerhans cells, keratinocytes, macrophages, and T cells.
  • UV radiation impairs T helper 1-mediated cellular immune response.

Knowledge of the interaction of sunlight with the skin is fundamental to understanding the pathogenesis, diagnosis, and treatment of more than 100 cutaneous disorders. Whenever ultraviolet (UV) or visible radiation is used to diagnose or treat a skin condition, important principles of photophysics involving absorption and emission of light underlie the success of the therapy. Sunscreen recommendations rely on an understanding of solar UV radiation and the ways in which the causative wavelengths can be minimized. Skin cancer is an epidemic clinical problem, whose pathophysiology necessitates comprehension of the photophysical, photochemical, and photobiologic events described in this chapter.

Almost every ancient civilization worshipped a god of the sun whose healing powers were believed to be broad reaching. Even today, sun exposure is widely felt to induce a sense of well-being. In addition, sunlight is important for the synthesis of vitamin D3 and the setting of internal clocks. On the negative side, sunlight causes deleterious acute and chronic inflammatory skin reactions, skin cancer, and photoaging, and can elicit adverse reactions to certain drugs (see Chapters 91, 92, 109, 112). Although the sun is a major source of the UV and visible radiation that interacts with human skin, UV and/or visible radiation are also emitted from common sources such as fluorescent lights, incandescent bulbs, photocopy machines, and phototherapy lamps. Tanning salons are another familiar example. Thus, UV and visible radiation are a constant part of the human environment and play a role in health, disease, and therapy. Photodermatology is the study ...

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